Download The time lag between a carbon dioxide emission and maximum

Environ. Res. Lett. 10 (2015) 031001
doi:10.1088/1748-9326/10/3/031001
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The time lag between a carbon dioxide emission and maximum
warming increases with the size of the emission
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24 January 2015
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9 February 2015
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Kirsten Zickfeld1 and Tyler Herrington1,2
1
2
Department of Geography, Simon Fraser University, Burnaby, B.C., Canada
Department of Geography, Douglas College, New Westminster, B.C., Canada
16 February 2015
E-mail: [email protected]
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Keywords: carbon dioxide emissions, warming commitment, ocean thermal inertia, Earth System Modelling
10 March 2015
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Abstract
In a recent letter, Ricke and Caldeira (2014 Environ. Res. Lett. 9 124002) estimated that the timing
between an emission and the maximum temperature response is a decade on average. In their analysis,
they took into account uncertainties about the carbon cycle, the rate of ocean heat uptake and the
climate sensitivity but did not consider one important uncertainty: the size of the emission. Using
simulations with an Earth System Model we show that the time lag between a carbon dioxide (CO2)
emission pulse and the maximum warming increases for larger pulses. Our results suggest that as CO2
accumulates in the atmosphere, the full warming effect of an emission may not be felt for several
decades, if not centuries. Most of the warming, however, will emerge relatively quickly, implying that
CO2 emission cuts will not only benefit subsequent generations but also the generation implementing
those cuts.
In a recent letter, Ricke and Caldeira [1] estimated that
the time lag between a carbon dioxide (CO2) emission
and the maximum warming response is a decade on
average. This is an important finding as it indicates that
the full climate damages expected to occur in response
to a CO2 emission will already be felt by the generation
responsible for those emissions. Conversely, the relatively short response timescale implies that CO2 emission cuts implemented today have the potential to
influence the rate of warming in the short term. Thus,
their finding corroborates the notion that the rate of
warming over the next decades is not inevitable, but will
be determined by future CO2 emissions [2].
A range of previous studies has explored the
warming commitment of past CO2 emissions [3–9]. A
robust finding of these studies is that the CO2-induced
warming persists for many centuries. The novel aspect
of the letter by Ricke and Caldeira [1] (henceforth
referred to as R&C) is its focus on the time lag between
a CO2 emission and the resulting maximum warming
response.
In their study, R&C used simple representations of
the carbon cycle and the physical climate system. The
carbon cycle was simulated by impulse response functions derived from the response of a range of Earth
System Models to a 100 GtC emission pulse [10]. The
© 2015 IOP Publishing Ltd
physical climate system was represented by onedimensional diffusion or two-box models fitted to
Coupled Model Intercomparison Project phase 5
(CMIP5) simulations with an abrupt quadrupling of
atmospheric CO2. Combinations of these models
made it possible to sample 6000 realizations of the
coupled climate-carbon cycle system.
For a 100 GtC pulse of CO2 released into the atmosphere with a background CO2 concentration of
389 ppm, R&C found the median time between an
emission and maximum warming to be 10.1 years,
with a 90% probability range of 6.6–30.7 years.
A pulse emission of CO2 results in an abrupt
increase in atmospheric CO2 concentrations, followed
by a slow decline as CO2 is taken up by the ocean and
terrestrial biosphere. The radiative forcing associated
with the CO2 increase causes warming, but the warming is delayed due to the long timescales of ocean heat
uptake. Initially, the ocean takes up a large amount of
heat, but this heat uptake decreases over time, allowing
the atmosphere to warm. The timing of peak warming
is determined by the balance between two opposing
processes: the decline of radiative forcing of atmospheric CO2 following the pulse emission (which has a
cooling effect on the atmosphere), and the decrease of
ocean heat uptake (which has a warming effect).
Environ. Res. Lett. 10 (2015) 031001
K Zickfeld and T Herrington
Figure 1. Global mean temperature response to CO2 emission pulses ranging from 100 to 5000 GtC as simulated by the UVic ESCM.
CO2 pulses were released into an atmosphere in equilibrium with pre-industrial atmospheric CO2 levels [10]. The temperature data
was normalized to facilitate comparison across simulations. The star symbols indicate the first temperature peak for each simulation.
The data from the 100 GtC pulse simulation was smoothed using a 5 year running average. The UVic ESCM pulse simulations were
conducted as part of the CO2-impulse response function model intercomparison project [10].
This balance is sensitive to a range of factors,
which differ widely across Earth System Models: the
sensitivity of the marine and terrestrial carbon sinks to
changes in atmospheric CO2 and climate, which determines the decline of atmospheric CO2; the rate of mixing of heat into the deep ocean, which determines the
decrease of ocean heat uptake; and the equilibrium climate sensitivity which also controls the response timescale of the system. R&C take these uncertainties into
account and show that uncertainty in the temperature
response timescale can only be narrowed if uncertainty in all three contributing factors is reduced.
One important factor that can be expected to affect
the timing of maximum warming, however, was not
considered in the uncertainty analysis: the size of the
CO2 emission pulse. As the pulse size increases, the
CO2 decline from peak levels slows [4]. The decline in
radiative forcing slows even more due to its logarithmic dependence on atmospheric CO2. At the same
time the decrease in ocean heat uptake is also reduced,
delaying the warming further. The combination of
these two processes has the potential to significantly
delay the time of peak warming for larger CO2 emission pulses.
Figure 1 shows the temperature response to pulse
emissions ranging from 100 GtC (the pulse size used in
the analysis of R&C) to 5000 GtC, as simulated with
the University of Victoria Earth System Climate Model
(UVic ESCM) [4, 11]. For a pulse emission of 100 GtC
the temperature response has a double-peak structure:
the first peak occurs in response to the abrupt increase
in radiative forcing, amplified by climate feedbacks.
The temperature decline from the first peak is caused
by the relatively fast decrease in atmospheric CO2 and
thus radiative forcing, which exceeds the decrease in
ocean heat uptake. The run-up to the second peak is
caused by the decrease in ocean heat uptake now
2
exceeding the decrease in radiative forcing. The second peak is reached when these two processes balance.
If the time-horizon of the analysis is limited to 100
years (as in R&C), the maximum warming occurs at
year 11, in agreement with the median of R&C. For
pulse emissions of 1000 GtC, the double-peak shape
remains but is less pronounced and the maximum
warming occurs 31 years after the emission. For very
large pulses (5000 GtC), the first peak disappears due
to the decline in ocean heat uptake overwhelming the
radiative forcing decline. In this case, the maximum
warming occurs 785 years after the emission.
The response time to larger pulses matters, as
future cumulative CO2 emissions are almost certain to
exceed 100 GtC. These cumulative emissions will arise
from successive small emission pulses, and the
response time for the total emission will be the
average response time for these individual pulses.
From our finding that the response time increases with
pulse size it follows that the response time of an individual CO2 emission is longer the later it is released
(i.e. the more CO2 has already accumulated in the
atmosphere).
Our results indicate that as CO2 continues to accumulate in the atmosphere, the full warming effect of an
emission may take several decades, if not centuries to
emerge. A large fraction of the warming, however, will
be realized relatively quickly (93% of the peak warming is realized 10 years after the emissions for the 1000
PgC pulse). This implies that the warming commitment from past CO2 emissions is small, and that future
warming will largely be determined by current and
future CO2 emissions. Each additional CO2 emission
will contribute to warming that will persist almost
indefinitely. Thus, emission reductions implemented
today will equally benefit current and future
generations.
Environ. Res. Lett. 10 (2015) 031001
K Zickfeld and T Herrington
Acknowledgments
The authors thank Michael Eby and H Damon
Matthews for insightful discussions. Kirsten Zickfeld
acknowledges support from the National Sciences and
Engineering Research Council of Canada (NSERC)
Discovery Grant Program.
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